A number of designs for mass transfer devices and separatory apparatuses using small capillary tubing made of semi-permeable membrane and methods for making the same are known. Among these designs and methods are those described in U.S. Pat. Nos. 2,972,349, 3,228,876, 3,228,877, 3,339,341, 3,422,008, 3,455,460, 3,475,331, 3,536,611, 3,579,810, 3,961,068, 3,704,223, 3,794,468, 3,883,024, and 4,881,813.
These mass transfer devices typically comprise a plurality of hollow capillay fibers made of semi-permeable membrane in a housing with appropriate inlet and outlet passages. The fibers generally extend from end to end within the housing. The exteriors of the fibers are encapsulated at opposite ends of the core by spaced bands of a rigid potting compound. The fibers have open ends so that fluid can be passed through them. The core is placed in a housing made of a casing and two end caps. Sealing means are provided so that a first fluid can flow through an opening in one end cap, through the fibers, and out the opposite end cap, while at the same time a second fluid can flow around the exterior of the fibers through openings in the casing.
One method of manufacturing these devices comprises attaching bundles of fibers to the potting compound. Another method involves wrapping a single fiber around a cylindrical core and then encapsulating the fiber with the potting compound at the opposite ends of the core. The ends of the fibers are then exposed by cuttng through the fibers and the potting material in a plane perpendicular to the axis of the core.
Although the above-described methods produce a useful separatory apparatus of the kind described above, they are not without disadvantages.
Chief among these disadvantages is lack of uniformity from unit to unit. Most of these units are essentially hand made. One unit, for example, is made up by taking a plurality of parallel fibers in a bundle with an outside diameter approximating the inside diameter of the housing and inserting the bundle in the housing. However, the fibers may be squeezed and twisted thereby causing distortion in the inside diameter of the fibers.
Another problem with many of these devices is that they are bulky due to the use of a large, cylindrical core. Large bulk tends to increase the cost of the device. Also those devices using cylindrical cores have a long spiral flow path which gives rise to high pressure drop and increased chance of plugging of the fibers.
Another disadvantage of existing devices is that spacing and density of the capillary fibers during assembly often is not closely controlled. This tends to increase the bulk of the device and give non-uniform flow through the fibers, with resultant poor mass transfer.
Another problem with many of the devices is that there is no means for directing the flow of the fluid flowing external to the fibers. Therefore channelling and dead spaces where no mass transfer occurs can result.
Another disadvantage of these devices is the means used for preventing the fluid inside the fibers from mixing with the fluid outside of the fibers. Typically a rigid potting compound is used to encapsulate the fibers in conjunction with an unsupported O-ring to provide a seal. However, the rigid sealing material tends to permanently deform when under pressure, thereby causing leaks.
Typically the end caps are either solvent or adhesive bonded to the casing. Although these methods provide a seal, it is a time consuming and expensive process because when solvent is used it must be allowed to completely dry, and when adhesive is used it must be allowed to cure. Furthermore, since pressure from the end cap typically is used to compress an O-ring, the caps must be held against the casing while the solvent is drying or the adhesive is curing. Another method of attaching end caps is by screw threads. However, there is a tendency for the threaded headers or end caps to back off with time and leaks develop.
Therefore, it is desirable to produce a compact mass transfer device which has a multiplicity of uniform fibers, has no dead spaces, has no fluid flow channelling, does not leak, and is inexpensive to manufacture.